Semiconductor devices are being made higher in density from year to year and the integrated circuit patterns formed on semiconductor wafers are becoming even finer. Exposure using a photomask is normally performed in forming an integrated circuit pattern on a semiconductor wafer, and the pattern on a photomask inevitably becomes finer as the pattern to be exposed becomes finer. Especially since the latter half of the 1990's, technical developments are actively being made towards forming fine shapes of sizes shorter than the light source wavelength of an exposure tool on a semiconductor wafer.
Generally in forming a fine pattern of a size near or no more than an exposure tool's light source wavelength on a semiconductor wafer, the diffraction phenomenon of light cannot be ignored. Specifically, in a case where a pair of mutually adjacent apertures are formed as a photo mask pattern, the lights transmitted through the pair of apertures diffract and interfere with each other, causing exposure even at parts that are supposed to be shielded from light. Thus with a photomask on which a fine pattern is formed, measures must be devised in consideration of the diffraction phenomenon of light. A phase shift mask is known as a type of photomask with which such a measure is taken. The basic principle of a phase shift mask is that a structure, with which the phases of the light transmitted through a pair of adjacently disposed apertures will be opposite each other, is employed to cancel out the interference of light. As a method of shifting the phase of light that is transmitted through one of the apertures by 180 degrees with respect to the phase of light that is transmitted through the other aperture, a method of forming a trench in the substrate that makes up the photomask has been proposed. For example, Laid-open Japanese Patent Publication No. 2002-40624 discloses a trench-type, Levenson-type phase shift mask as a typical example of such a phase shift mask.
As mentioned above, with a phase shift mask, since the shape of a fine pattern must be determined in consideration of the diffraction phenomenon of light, the design work is complicated. Especially with a trench-type, Levenson-type phase shift mask, in which a trench is formed in a substrate, since the part at which the trench is formed takes on a three-dimensional structure, two-dimensional analysis is insufficient and the need to perform three-dimensional analysis arises. Much labor and time were thus required to design a single phase shift mask.
An object of this invention is thus to provide a designing method and device for phase shift mask which enable the work load to be lightened and the working time to be shortened.